Special request: "double depth maps"
Here is an example setup for rendering both front and back depth using red and blue channels with Blender Cycles. 

We are rendering "depth" through Cycles emission that mixes with transparency so we can receive emission from the backfacing surface as well. In this setup the emission value is derived from world space Z-coordinate and orthographic camera is facing down.



Red emission receives depth as is, so a face that has Z-coordinate of 0 is black and 1 is fully red. Red depth is multiplied by inverse of Transparent Depth so we are only receiving it from the front surfaces where the value is 0 and inverse is 1.

Blue emission receives inverse of depth, so a face that has Z-coordinate of 1 is black and 0 is fully blue. Blue depth is multiplied by the Transparent Depth so we are only receiving it from the backfacing surfaces where the value is 1.